Evolution of O Abundance Relative to Fe

Abstract

We present a three-component mixing model for the evolution of O abundance relative to Fe, taking into account the contributions of the first very massive stars (with masses of ≳100 M_☉) formed from Big Bang debris. We show that the observations of O and Fe abundances in metal-poor stars in the Galaxy by Israelian et al. in 1998 and by Boesgaard et al. in 1999 can be well represented both qualitatively and quantitatively by this model. We use the representation of the number ratios (O/Fe) versus 1/(Fe/H). In this representation, if there is only a single source with a fixed production ratio of O to Fe beyond a certain point, the subsequent evolution of (O/Fe) is along a straight line segment. Under the assumption of an initial Fe ([Fe/H] ~ -3) and O inventory caused by the prompt production by the first very massive stars, the data of Israelian et al. and Boesgaard et al. at -3≾[Fe/H]≾-1 are interpreted to result from the addition of O and Fe only from Type II supernovae (SNII) to the prompt inventory. At [Fe/H] ≳-1, SNII still contribute O while both SNII and Type Ia supernovae contribute Fe. During this later stage, (O/Fe) sharply drops off to an asymptotic value of ~0.8 (O/Fe)_☉. The value of (O/Fe) for the prompt inventory at [Fe/H] ~ -3 is found to be (O/Fe) ~ 20 (O/Fe)_☉. This result suggests that protogalaxies with low "metallicities" should exhibit high values of (O/Fe). The C/O ratio produced by the first very massive stars is expected to be « 1 so that all the C should be tied up as CO and that C dust and hydrocarbon compounds should be quite rare at epochs corresponding to [Fe/H] ≾ -3.